Determination of equivalent series resistance

ABSTRACT

A determination of an equivalent series resistance (ESR) effect for high frequency filtering performance of a filtered feed-through assembly is described. A low frequency signal is introduced to a filtered feed-through assembly. ESR limit of the filtered feed-through is determined based on the low frequency signal.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/236,369, filed Sep. 27, 2005 entitled “Determination of EquivalentSeries Resistance”. This application is also related to U.S. patentapplication Ser. No. 11/235,944 filed on Sep. 27, 2005 and entitled“DETERMINATION OF EQUIVALENT SERIES RESISTANCE”.

FIELD OF THE INVENTION

The present invention relates to measurement techniques and, morespecifically, to determination of equivalent series resistance (ESR)effect on high frequency filtering performance of a filteredfeed-through assembly.

BACKGROUND OF THE INVENTION

Implantable medical devices (IMDs) detect and correct a variety ofmedical conditions in patients. Exemplary IMDs include implantable pulsegenerators (IPGs) or implantable cardioverter-defibrillators (ICDs) thatdeliver electrical stimulation to tissue of a patient. IMDs typicallyinclude, inter alia, a control module, a capacitor, and a battery. Thesecomponents are housed in a container that is hermetically sealed toprevent liquid from contacting the electronic components therein. Toconnect with the electronic components, a feed-through assembly iscoupled to the container. The feed-through assembly forms an aperture inthe container, which allows an electrical lead to pass therethrough.

Electromagnetic interference (EMI) may affect the operation of an IMD.EMI is any electromagnetic disturbance, phenomenon, signal, or emissionthat causes or can cause an undesired response in an IMD. To addressEMI, an EMI filter is incorporated into a feed-through assembly. Atypical filtered feed-through assembly consists of a conductive line, aferrule, an insulator member (e.g. glass, ceramic etc.), at least onecapacitor, and a seal. The filtered feed-through assembly is configuredsuch that the seal lines an aperture located in the ferrule. Theinsulator member is placed in the aperture adjacent to the seal. Theconductive line is connected to the ferrule and the capacitor. A lead(e.g. wire) or a terminal pin to the sealed container is insertedthrough another aperture in the insulator member, which provides anelectrical connection to the components within the IMD.

The filtered feed-through assembly capacitor acts as a low pass filterto prevent EMI from affecting the operation of the IMD. A low passfilter allows low frequency signals to pass but prevents high frequencysignals from passing therethrough. The performance of the EMI filter isdetermined by insertion loss. Insertion loss results from the insertionof a device in a transmission line, expressed as the reciprocal of theratio of the signal power delivered to that part of the line followingthe device to the signal power delivered to that same part beforeinsertion. Insertion loss depends upon the number of components in theEMI filter, impedance value of each EMI filter component, the frequencyat which the insertion loss is measured, equivalent series resistance(ESR), and equivalent series induction (ESL).

Insertion loss measurement at high frequencies is difficult due toincreased noise detected for high frequency signals. To determineinsertion loss at a high frequency, a radio frequency (RF) shield iswelded to the filtered feed-through assembly. The RF shield isolates theinput and output ends of the filtered feed-through assembly during thetest. The welding operation and the RF shield itself increase the costof producing an IMD. Additionally, this insertion loss measurementmethod cannot be used on a large-scale basis.

Alternatively, insertion loss may be measured without a RF shield weldedto the filtered feed-through assembly. However, this type of insertionloss measurement is unreliable. It is therefore desirable to overcomethe limitations associated with conventional testing systems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of an exemplary testing system;

FIG. 2 is a perspective side view of an exemplary feed-through assembly;

FIG. 3 is a cross-sectional view of an exemplary filtered feed-throughassembly;

FIG. 4 is a perspective side view of an exemplary test fixture and adevice under test;

FIG. 5 is a portion of an exemplary test fixture with a pin piercing anoxide layer to connect with a ferrule of a filtered feed-throughassembly;

FIG. 6 is a graph illustrating insertion loss versus equivalent seriesresistance for a device evaluated by the testing system of FIG. 1; and

FIG. 7 is a flow diagram of a method to predict or estimate insertionloss associated with a feed-through assembly.

DETAILED DESCRIPTION

The following description of an embodiment is merely exemplary in natureand is in no way intended to limit the invention, its application, oruses. For purposes of clarity, the same reference numbers are used inthe drawings to identify similar elements. As used herein, the term“module” refers to an application specific integrated circuit (ASIC), anelectronic circuit, a processor (shared, dedicated, or group) and memorythat execute one or more software or firmware programs, a combinationallogic circuit, or other suitable components that provide the describedfunctionality.

Generally, the present invention is directed to measurement of insertionloss related to a device (e.g., filtered feed-through assembly) over acertain frequency range. For example, the frequency range of about 450megahertz (MHz) to about 3 gigahertz (GHz) is used to determine theeffectiveness of the low pass filter (e.g., capacitor(s) in the filteredfeed-through assembly) disposed in an implantable medical device (IMD).This insertion loss measurement technique may be used over any otherappropriate frequency range for a particular device.

Specifically, the present invention relates to measurement of equivalentseries resistance (ESR) effect on high frequency filtering performanceof a filtered feed-through assembly for a device. A set of insertionloss data at a first high frequency (e.g. 450 MHz to about 3 GHz) isdetermined for a filtered feed-through assembly. A set of insertion lossdata is generated at a second high frequency (e.g. 2450 MHz) for thefiltered feed-through assembly. A set of ESR data is generated at a lowfrequency (e.g. 2 MHz) for the filtered feed-through assembly. The setof ESR data at the low frequency is correlated to the insertion lossgenerated at a first high frequency. This correlation may be graphicallyrepresented or tabulated. A prediction interval that includes upper andlower limits is superimposed on the graph. A specified insertion lossdata (e.g. 30 dB) is selected for a particular device. An ESR limit fora low frequency signal is determined based upon the specified insertionloss data and the upper limit of the prediction interval. The ESR limitis compared to ESR values of manufactured filtered feed-throughassemblies. Filtered feed-through assemblies are of acceptable qualitywhen their ESR value is below the ESR limit. In contrast, filteredfeed-through assemblies are rejected that exhibit an ESR value above theESR limit.

The present invention increases the accuracy of determining the ESReffect at high frequency signals filtered via a filtered feed-throughassembly. Additionally, the present invention reduces the cost ofproducing an implantable medical device by eliminating welding of ashield to a feed-through assembly. Furthermore, the present inventionmay be implemented on a mass production scale.

FIG. 1 depicts a test system 10 that determines ESR effect on highfrequency filtering performance of a device under test (DUT). Testsystem 10 includes a control module 12, a switch matrix 14, aninductance, capacitance, resistance (LCR) meter or impedance meter 16(i.e. measurement meter), and a test fixture 18.

Control module 12 is typically a computer that includes a parallel port(not shown) connected to bus 20 a. The parallel port generally includes32 lines and 16 input/output ports. Since two lines are associated witha single input/output port, multiple busses may be connected to theparallel port. Control module 12 connects to LCR meter 16 and switchmatrix 14 via buses 20 a, 20 b, and 20 c respectively. Control module 12inputs a control signal over buses 20 a, 20 b to LCR meter 16. LCR meter16 is configured to measure and transmit to control module 12 theinductance, capacitance, and resistance associated with a DUT that isconnected to test fixture 18.

In one embodiment, the DUT is a filtered feed-through assembly 30depicted in FIGS. 2-3. Filtered feed-through assembly 30 consists of oneor more conductive lines 25 a-b that include nonpolarized capacitors 26,a ferrule 32, and an insulator member (e.g. glass, ceramic etc.) 33.FIGS. 4-5 depict test fixture's 18 set of pogo pins 34 include spearpoints 38 that are configured to pierce an oxide layer 36. At least onepogo pin 34 directly contacts ferrule 32 to securely form a groundconnection. The secure ground connection assists in detecting andmeasuring more accurate data than conventional systems. In addition toproviding accurate insertion loss and ESR data, test system 10 alsoprovides more accurate data related to measurement of capacitance,displacement factors, high voltage resistance, and other suitablefactors.

LCR meter 16 inputs a test signal at a certain frequency to switchmatrix 14 over conductive lines PH, IH. PH is a positive voltageconductive line whereas IH is a positive current conductive line. Basedupon the signal from LCR meter 16, switch matrix 14 selects one of fourchannels to test one or more conductive lines 25 a-d of a feed-throughassembly 30. A first channel is formed by conductive lines N-7 and N-6;a second channel is formed by conductive lines N-5 and N-4; a thirdchannel is formed by conductive lines N-3 and N-2; and a fourth channelis formed by conductive lines N-1 and N. Switch matrix 14 then signals aconductive line 25 a over the selected channel. A signal is thentransmitted from conductive line 25 a to LCR meter 16 over negativevoltage conductive line PL and negative current conductive line IL. LCRmeter 16 determines the ESR for test fixture 18 and the insertion lossat two different high frequencies (e.g. 450 megahertz (MHz) and at 2450MHz etc.). Generally, a high frequency ranges from about 9 KHz to about10,000 MHz. The ESR is also measured at 2 MHz for the filteredfeed-through assembly. The data is transmitted to control module 12.This process is repeated for a set of filtered feed-through assemblies.Control module 12 then executes regression analysis instructions thatcorrelate insertion loss data obtained at a high frequency to ESR dataobtained at a low frequency, as depicted in the graph of FIG. 6.Exemplary regression analysis software is commercially available fromMinitab located in State College, Pa. Regression analysis is describedin detail by W. Mendenhall and T. Sincich, Statistics For EngineeringAnd the Sciences (4th Ed. 1995), which is incorporated by reference, inrelevant parts.

An example of simple regression analysis is presented below. Regressionanalysis determines the prediction interval (PI) for insertion loss, asdepicted in FIG. 6. PI, defined by lower and upper limits 40, 42, is aninterval that quantifies the degree of uncertainty in a futureprediction relative to insertion loss of a DUT. The formula to calculatethe PI for insertion loss, defined as y, is as follows:

$\hat{y} \pm {{t\left( {{n - 2},\frac{\alpha}{2}} \right)} \cdot s_{e} \cdot \sqrt{1 + \frac{1}{n} + \frac{\left( {x_{0} - \overset{\_}{x}} \right)^{2}}{SSx}}}$where,t is the Student's t-distribution statistic obtained from tabulateddata,t (df,α) is the critical value for the Student's t-distribution,df=(n−2) is the degree of freedom,=probability of type 1 error=0.01,n=sample size (i.e. number of pieces of data in one sample)x=is the average value for x,X₀=ESR measured at a low frequency,

$s_{e}^{2} = \frac{\left( {\sum y^{2}} \right) - {\left( b_{0} \right)\left( {\sum y} \right)} - {\left( b_{1} \right)\left( {\sum{xy}} \right)}}{n - 2}$$b_{1} = \frac{\sum{\left( {x - \overset{\_}{x}} \right)\left( {y - \overset{\_}{y}} \right)}}{\sum\left( {x - \overset{\_}{x}} \right)^{2}}$y=the average value of insertion loss data

$b_{0} = \frac{{\sum y} - \left( {b_{1} \cdot {\sum x}} \right)}{n}$ŷ=predicted value of y for a given x

${SSx} = {{\sum x^{2}} - \frac{\left( {\sum x} \right)^{2}}{n}}$

The lower and upper PI limits 40, 42 are determined from the confidencelevel and the standard error of the prediction. The PI is generallywider than a confidence interval because of the added uncertaintyinvolved in predicting a single response versus the mean response. ThePI is a discrete determination that is performed at a chosen Xo over adesired ESR range to create a curve. In the present example, Xo isselected over an ESR range such that the upper limit 40 for insertionloss crosses the desired insertion loss specification limit 44 (Y-axis).In this example, the desired insertion loss limit is −30 decibels (dB).From this data, the ESR limit 46 is determined to be 2.8Ω since this isthe point at which upper limit 42 crosses the desired insertion losslimit 44.

Using the above formula, the value of the Y interval (i.e., insertionloss) is predicted in an iterative manner for a given X₀ (ESR). Table 1summarizes ESR data at a lower frequency (e.g. 2 MHz) and the upper andlower limits for insertion loss.

TABLE 1 Tabulated data for a filtered feed-through assembly PredictionLimit For Prediction Limit For X₀ (ESR Insertion Loss at 450 MHzInsertion Loss at 450 MHz at 2 MHz) Ω (Lower Limit) (Upper Limit) 2.3−43.9578 −33.9257 2.4 −43.1368 −33.1196 2.5 −42.3326 −32.3272 2.6−41.5451 −31.5485 2.7 −40.7741 −30.7837 2.75 −40.3948 −30.4065 2.8−40.0196 −30.0329 2.85 −39.6484 −29.6628 2.9 −39.2813 −29.2963 2.95−38.9182 −28.9333 3 −38.5592 −28.5739

The ESR limit is based on a minimum insertion loss requirement. ESRlimit 46 is used to accept or reject a part. In particular, ESR limit 46is compared to the ESR exhibited by the DUT. If the ESR of the DUT isbelow ESR limit 46, the DUT satisfies quality specifications. However,if the DUT's ESR value exceeds ESR limit 46, the DUT does not meetquality specifications. DUT's are rejected that exceed ESR limit 46. Inaddition to using the ESR limit 46 to determine the quality of a DUT,the ESR value measured on the DUT can also be used to predict theinsertion loss of the DUT using the same method.

FIG. 6 is a flow diagram of one method to determine an ESR limit at alow frequency. At operation 110, a set of insertion loss data at a firsthigh frequency (e.g. 450 MHz) is determined for a filtered feed-throughassembly. At operation 120, a set of insertion loss data is generated ata second high frequency (e.g. 2450 MHz) for the filtered feed-throughassembly. At operation 130, a set of ESR data is generated at a lowfrequency (e.g. 2 MHz) for the filtered feed-through assembly. The setof ESR data at a low frequency is correlated to the insertion lossgenerated at a first high frequency. This correlation may be graphicallyrepresented or tabulated. At operation 140, the set of insertion lossdata is correlated to the set of ESR data. At operation 150, aprediction interval that includes upper and lower limits is generatedand superimposed on the graph. At operation 160, a specified insertionloss value (e.g. 30 dB) is selected. The specified insert in loss valueis based upon the desired minimal quality that is sought for amanufactured device (e.g. filtered feed-through assembly). At operation170, an ESR limit for a low frequency signal is determined. The ESRlimit is based upon the first insertion loss data and the upper limit ofthe prediction interval. The ESR limit is then used to reject or acceptmanufactured feed-through assemblies.

The present invention has numerous applications. For example, while thefigures relate to quadripolar filtered feed-through assemblies, othertypes of feed-through assemblies may also rely on this process toreliably produce quality feed-through assemblies. Additionally, theoperation described herein may be implemented entirely automatically ormanually. Moreover, skilled artisans appreciate that measurementtechniques are not polarity sensitive. Accordingly, techniques of theinvention may be implemented with PH having a negative voltage and PLhaving a positive voltage. Similarly, IH can possess a negative currentand IL can be a positive current. Furthermore, while the presentinvention describes rejection of a manufactured device (e.g. filteredfeed-through assembly etc.) that exceeds an ESR limit, skilled artisansunderstand that principles of the invention may also be configured in amanner such that a manufactured device is rejected that is below an ESRlimit.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A test system to determine if a filtered feed-through assembly is ofacceptable quality, wherein the filtered feed-through assembly includesone or more conductive lines and a ferrule, the test system comprising:a test fixture comprising a plurality of pins electrically connectableto a filtered feed-through assembly, wherein each pin includes a sharpdistal tip, and further wherein the plurality of pins comprises at leasta conductor probe pin and at least a ground probe pin, the conductorprobe pin being electrically connectable to one of the one or moreconductive lines and the ground probe pin being electrically connectableto the ferrule to provide a ground connection; a measurement meterelectrically connectable via the conductor probe pin of the test fixtureto a filtered feed-through assembly under test and electricallyconnectable via the ground probe pin of the test fixture to the filteredfeed-through assembly under test, wherein the measurement meter iscontrollable to transmit signals at one or more frequencies to beapplied to the filtered feed-through assembly under test and to measureone or more electrical parameters of the filtered feed-through assemblyunder test for use in determining at least equivalent series resistance(ESR) data and insertion loss data for the filtered feed-throughassembly under test; and a control module coupled to the measurementmeter to control measurement of the one or more electrical parameters ofthe filtered feed-through assembly under test, and further wherein thecontrol module comprises a processor operable to execute instructions tocorrelate ESR data to insertion loss data for one or more filteredfeed-through assemblies and to determine an ESR limit based on thecorrelation and a selected desired insertion loss limit, wherein the ESRlimit is used to determine if a filtered feed-through assembly is ofacceptable quality, wherein the sharp distal tip of at least the groundprobe pin is configured to pierce an oxide layer over a ferrule of afiltered feed-through assembly under test to securely form the groundconnection.
 2. The test system of claim 1 wherein the conductor probepin and the ground probe pin of the plurality of pins directly contactone of the conductive lines and the ferrule, respectively, of thefiltered feed-through assembly during measurement of the one or moreelectrical parameters of the filtered feed-through assembly under test.3. The test system of claim 1 wherein the measurement meter iscontrollable to provide an alternating current signal at one or morefrequencies to the filtered feed-through assembly under test duringmeasurement of the one or more electrical parameters of the filteredfeed-through assembly under test.
 4. The test system of claim 1 whereinthe processor of the control module is operable to execute instructionsstored in memory to: determine insertion loss data at a high frequency;determine a set of ESR data at a low frequency; determine an ESR limitat a low frequency; determine an ESR value for a manufactured filteredfeed-through assembly; and determine quality of the manufacturedfiltered feed-through assembly based upon the ESR limit.
 5. The testsystem of claim 1 wherein the test system further comprises a pluralityof conductor probe pins arranged to contact a plurality of conductivelines of a filtered feed-through assembly, and further wherein the testsystem comprises a switch matrix electrically coupled to the pluralityof conductor probe pins of the test fixture via a plurality ofselectable channels and also electrically coupled to the measurementdevice, wherein the control module is coupled to the switch matrix tocontrol selection of one of the plurality of selectable channels toselectively couple the measurement meter to a selected conductor probepin.
 6. The test system of claim 1 wherein the processor is operable toexecute instructions to apply regression analysis to correlate ESR dataand insertion loss data.
 7. The test system of claim 1 wherein theprocessor is operable to execute instructions to correlate ESR data andinsertion loss data for a set of filtered feed-through assemblies and todetermine an ESR limit based on the correlation and on a selecteddesired insertion loss limit.
 8. The test system of claim 1 wherein theprocessor is operable to execute instructions to: determine insertionloss data for a set of filtered feed-through assemblies based upon afirst high frequency signal; determine insertion loss data for the setof filtered feed-through assemblies based upon a second high frequencysignal; determine ESR data for the set of filtered feed-throughassemblies based upon a low frequency signal; apply regression analysisto the ESR data and insertion loss data; create a prediction intervalfor insertion loss defined by an upper limit and a lower limit; acquirethe selected desired insertion loss limit representative of the desiredminimal quality that is sought for a manufactured filtered feed-throughassembly; and determine the ESR limit based on the selected desiredinsertion loss limit and the upper limit defined for the predictioninterval.
 9. A method comprising: correlating, with a processorexecuting instructions, ESR data and insertion loss data for one or morefiltered feed-through assemblies and determining an ESR limit at a lowfrequency based on the correlation and a selected desired insertion losslimit; determining an ESR value for a manufactured filtered feed-throughassembly; and determining quality of the manufactured filteredfeed-through assembly based on a comparison of the ESR value to the ESRlimit.
 10. The method of claim 5 further comprising: forming a secureground connection between a ferrule of the manufactured filteredfeed-through assembly and a pin of a test fixture used to determine theESR value.
 11. The method of claim 9 further comprising: rejecting themanufactured feed-through assembly if the ESR value exceeds the ESRlimit.
 12. The method of claim 9, wherein correlating, with theprocessor executing instructions, ESR data and insertion loss data forone or more filtered feed-through assemblies comprises applyingregression analysis to ESR data and insertion loss data.
 13. The methodof claim 9, wherein correlating, with the processor executinginstructions, ESR data and insertion loss data for one or more filteredfeed-through assemblies comprises correlating ESR data and insertionloss data for a set of filtered feed-through assemblies.
 14. The methodof claim 9, wherein correlating, with the processor executinginstructions, ESR data and insertion loss data for one or more filteredfeed-through assemblies comprises: determining insertion loss data for aset of filtered feed-through assemblies based upon a first highfrequency signal; determining insertion loss data for the set offiltered feed-through assemblies based upon a second high frequencysignal; determining ESR data for the set of filtered feed-throughassemblies based upon a low frequency signal; applying regressionanalysis to the ESR data and insertion loss data; creating a predictioninterval for insertion loss defined by an upper limit and a lower limit;acquiring the selected desired insertion loss limit representative ofthe desired minimal quality that is sought for a manufactured filteredfeed-through assembly; and determining the ESR limit based on theselected desired insertion loss limit and the upper limit defined forthe prediction interval.
 15. A method comprising: correlating, with aprocessor executing instructions, ESR data and insertion loss data for aset of filtered feed-through assemblies and determining an ESR limit ata low frequency based on the correlation and a selected desiredinsertion loss limit; determining an ESR value for a manufacturedfiltered feed-through assembly; determining quality of the manufacturedfiltered feed-through assembly based on a comparison of the ESR value toupon the ESR limit; forming a secure ground connection between a ferruleof the manufactured filtered feed-through assembly and a pin of a testfixture used to determine the ESR value; and rejecting the manufacturedfeed-through assembly if the ESR value exceeds the ESR limit.
 16. Themethod of claim 15 wherein correlating, with the processor executinginstructions, ESR data and insertion loss data for the set of filteredfeed-through assemblies comprises: determining insertion loss data for aset of filtered feed-through assemblies based upon a first highfrequency signal; determining insertion loss data for the set offiltered feed-through assemblies based upon a second high frequencysignal; determining ESR data for the set of filtered feed-throughassemblies based upon a low frequency signal; applying regressionanalysis to the ESR data and insertion loss data; creating a predictioninterval for insertion loss defined by an upper limit and a lower limit;acquiring the selected desired insertion loss limit representative ofthe desired minimal quality that is sought for a manufactured filteredfeed-through assembly; and determining the ESR limit based on theselected desired insertion loss limit and the upper limit defined forthe prediction interval.